The ability to quantify spatially discrete dopamine (DA) concentration over a chronic, multi-week timescale is paramount to unlocking the mechanisms underlying healthy and disease state behavior. DA signaling throughout the brain occurs over multiple timescales. Phasic signaling results from high frequency burst firing, whereas tonic DA release is maintained by low frequency ?pacemaker firing?. For decades, fast scan cyclic voltammetry (FSCV) at carbon fiber microelectrodes (CFEs) has been used to record sub second phasic DA transmission, but measuring resting level, tonic DA concentrations has been a technical challenge. We have recently shown that conductive polymer nanocomposite coating consisting of poly(3,4-ethylenedioxythiophene) and acid functionalized carbon nanotubes (PEDOT/CNT) significantly increases electrode sensitivity and selectivity for DA, and when combined with a novel square wave voltammetry (SWV) protocol, is capable of measuring resting DA concentrations in vivo with high selectivity. The same coating can also record sub second DA release using FSCV. Microfabricated multielectrode arrays (MEAs) have been developed to monitor neurophysiological signals simultaneously from multiple recording sites with high spatial resolution. However, the poor sensitivity and selectivity of conventional metal electrodes have limited the use of MEAs for neurochemical sensing. By applying PEDOT/fCNT coating onto MEAs, we can increase both the sensitivity and selectivity of neurochemical detection from MEA. Furthermore, implantation of stiff MEAs inevitably causes neuronal damage and inflammatory glial response, both of which compromise sensor performance, especially for long-term applications. Recent advancement in MEA technology has revealed that flexible and subcellular sized implants significantly mitigate the foreign body response resulting in seamless integration within neural tissue. Here, we hypothesize that chronic multisite DA measurement can be enabled through combining the highly sensitive PEDOT/CNT coating with ultra-small, flexible neural recording probe technology. The first specific aim is to fabricate PEDOT/fCNT functionalized flexible MEA capable of detecting DA with sensitivities and LODs in the physiologically relevant concentration range. PEDOT/CNT coating conditions will be optimized for electrode sites on 16-channel, flexible SU-8 MEAs. DA sensing performance will be investigated using SWV and FSCV in the presence of interferents. Coating stability will be assessed via mechanical bending and agar insertion experiments. The second specific aim is to determine the efficacy of PEDOT/fCNT functionalized flexible MEAs for acute and chronic in vivo DA sensing. In the acute validation experiments, sensors will be implanted into the DS of anesthetized rats. SWV (measure basal DA) and FSCV (measure electrically evoked sub second DA) measurements will be recorded from 16 individual electrode sites spanning the entire sagittal length of the DS (4 mm) before and after the acute administration of either nomifensine (NOM, increase DA) or ?-methyl-DL-tyrosine (?MPT, decrease DA). In the chronic experiments, sensors will be used to record spatially discrete tonic and electrically evoked phasic DA over 28 days in 6-OH-DA lesioned rats over a period of 28 days post lesion. Post-mortem immunohistology will be conducted after 7, 14 and 28 days of probe implantation to assess inflammatory host tissue response and to monitor lesion formation. Observation of any spatially correlated changes in resting and electrically evoked DA relating to 6-OH-DA lesion formation will reveal the effectiveness of the chronic DA sensor and provide valuable physiological insight into Parkinson's Disease. This proposal has the potential to revolutionize the state of the art of neurochemical sensing by enabling high fidelity chronic measurement of tonic and phasic DA release from multiple discrete neuron groupings. The proposed in vivo experiments could shed light on the mechanisms of Parkinsonian DA compensation. On a broader sense, this technology will find a wide spread use throughout a range of basic neuroscience and clinical research areas with the ultimate goal of understanding healthy and diseased brain as well as developing effective therapies.